Blend membranes based on sulfonated poly(phenylene oxide) for polymer electrochemical cells

ABSTRACT

Solid polymer membranes comprised of a high charge density sulfonated poly (phenylene oxide) blended with poly(vinylidene fluoride) in varied ratios have improved membrane characteristics. These membranes are inexpensive and possess very high ionic conductivity, and thus are suitable for solid polymer electrolytes in electrochemical applications, especially for the polymer electrolyte membrane (PEM) fuel cell, the electrolyte double-layer capacitor, and the rechargeable zinc-halide cell. These membranes enhance the performance of these devices.

This application is a continuation-in-part of Ser. No. 08/725,747, filedOct. 4, 1996, now U.S. Pat. No. 5,989,742, which is acontinuation-in-part of Ser. No. 08/580,381, filed Dec. 28, 1995, nowabandoned.

FIELD OF THE INVENTION

This invention relates to homogeneous blends of sulfonatedpoly(phenylene oxide) and poly(vinylidene fluoride), their use as ionexchange membranes in electrochemical applications, such as solidpolymer electrolyte fuel cells, electrolyte double-layer capacitors, andrechargeable zinc-halide cells. Further, this invention also relates toan improved solid polymer electrolyte fuel cell containing the novelblend membranes. This invention also relates to an improved electrolytedouble-layer capacitor containing the novel blend membranes.Additionally, the invention relates to a rechargeable zinc-halide cellcontaining the novel blend membranes.

BACKGROUND OF THE INVENTION

Fuel cells are electrochemical devices in which part of the energy of achemical reaction is converted directly into direct current electricalenergy. The direct conversion of energy into direct current electricalenergy eliminates the necessity of converting energy into heat therebyavoiding the Carnot-cycle efficiency limitation of conventional methodsof generating electricity. Thus, without the limitation of theCarnot-cycle, fuel cell technology offers the potential for fuelefficiencies two to three times higher than those of traditional powergenerator devices, e.g., internal combustion engines. Other advantagesof fuel cells are quietness, cleanliness (lack of air pollution) and thereduction or the complete elimination of moving parts.

Typically, fuel cells contain two porous electrical terminals calledelectrodes with an electrolyte disposed therebetween. In the operationof a typical fuel cell, an oxidant is continuously introduced at theoxidant electrode (cathode) where it contacts the electrode and formsions thereby imparting positive charges to the cathode. Simultaneously,a reductant is continuously introduced at the fuel electrode (anode)where it forms ions and leaves the anode negatively charged. The ionsformed at the respective electrodes migrate in the electrolyte and unitewhile the electrical charges imparted to the electrode are utilized aselectrical energy by connecting an external circuit across theelectrodes. Fuel cell reactants are classified as oxidants andreductants based on their electron acceptor or electron donorcharacteristics. Oxidants include pure oxygen, oxygen-containing gases(e.g., air) and halogens (e.g., chlorine). Reductants include hydrogen,carbon monoxide, natural gas, methane, ethane, formaldehyde andmethanol.

The electrolyte of the fuel cell serves as the electrochemicalconnection between the electrodes providing a path for ionic current inthe circuit while the electrodes, made of carbon or metal, provide anelectrical pathway. Further, the electrolyte prevents transfer of thereactants away from the respective electrodes where the formation ofexplosive mixtures can occur. The electrolyte utilized must not reactdirectly to any appreciable extent with the reactants or reactionproducts formed during the operation of the fuel cell. Further, theelectrolyte must permit the migration of ions formed during operation ofthe fuel cell. Examples of electrolytes that have been used are aqueoussolutions of strong bases, such as alkali metal hydroxides, aqueoussolutions of acids, such as sulfuric acid and hydrochloric acid, aqueoussalt electrolytes, such as sea water, fused salt electrolytes andion-exchange polymer membranes.

One type of fuel cell is a polymer electrolyte (PEM) fuel cell which isbased on a proton exchange polymer membrane. The PEM fuel cell containsa solid polymer membrane, which is an "ion-exchange membrane" that actsas an electrolyte. The ion-exchange membrane is sandwiched between two"gas diffusion" electrodes, an anode and a cathode, each commonlycontaining a metal catalyst supported by an electrically conductivematerial. The gas diffusion electrodes are exposed to the respectivereactant gases, the reductant gas and the oxidant gas. Anelectrochemical reaction occurs at each of the two junctions (threephase boundaries) where one of the electrodes, electrolyte polymermembrane and reactant gas interface.

For example, when oxygen is the oxidant gas and hydrogen is thereductant gas, the anode is supplied with hydrogen and the cathode withoxygen. The overall chemical reaction in this process is: 2H₂ +O₂ →2H₂O. The electrochemical reactions that occur at the metal catalyst sitesof the electrodes are as follows:

anode reaction: 2H₂ →4H⁺ +4e⁻

cathode reaction: O.sub. +4H⁺ +4e⁻ →2H₂ O

During fuel cell operation, hydrogen permeates through the anode andinteracts with the metal catalyst, producing electrons and protons. Theelectrons are conducted via an electronic route through the electricallyconductive material and the external circuit to the cathode, while theprotons are simultaneously transferred via an ionic route through thepolymer electrolyte membrane to the cathode. Concurrently, oxygenpermeates to the catalyst sites of the cathode, where the oxygen gainselectrons and reacts with the protons to yield water. Consequently, theproducts of the PEM fuel cell reactions are water and electricity. Inthe PEM fuel cell, current is conducted simultaneously through ionic andelectronic routes. Efficiency of the PEM fuel cell is largely dependenton the ability to minimize both ionic and electronic resistivity tocurrent.

Ion-exchange membranes play a vital role in PEM fuel cells. Improvedmembranes have substantially increased power density. In PEM fuel cells,the ion-exchange membrane has two functions: (1) it acts as theelectrolyte that provides ionic communication between the anode andcathode; and (2) it serves as a separator for the two reactant gases(e.g., O₂ and H₂).

Optimized proton and water transports of the membrane and proper watermanagement are crucial for efficient fuel cell application. Dehydrationof the membrane reduces proton conductivity, and excess water can leadto swelling of the membranes and flooding of the electrodes. Bothconditions result in poor cell performance. In the fuel cell, theion-exchange membrane, while serving as a good proton transfer membrane,also must have low permeability for the reactant gases to avoidcrossover phenomena that reduce performance of the fuel cell. This isespecially important in fuel cell applications in which the reactantgases are under pressure and the fuel cell is operated at elevatedtemperatures. Therefore, a good ion-exchange membrane for a PEM fuelcell has to meet the following criteria: (1) chemical andelectrochemical stability in the fuel cell operating environment; (2)mechanical strength and stability under cell operating conditions; (3)high proton conductivity, low permeability to reactant gas, and highwater transport; and (4) low production costs.

An electrolyte double-layer capacitor comprises a separator layerpositioned between polarizable electrodes. Almost every double-layercapacitor uses either filter paper or thin prose polypropylene film.Polypropylene (PP) is very hydrophobic with high electric resistance,which results in an increased internal resistance of the double-layercapacitor. Therefore, a good ion-exchange membrane for an electrolytedouble-layer capacitor has to meet the following criteria: (1) chemicaland electrochemical stability in the operating environment; (2)mechanical strength and stability under operating conditions; (3) highion conductivity; (4) low electric resistance, and (5) low productioncosts.

Zinc-halogen rechargeable cells have a high theoretical energy density.In those cells, an aqueous zinc halide salt electrolyzes to zinc metal,which deposits on an anode, while molecular halides accumulate at acathode. As the cell discharges, zinc and molecular halide react backinto the salt form. However, diffusion of the halides from the cathodeto the anode causes internal self-discharge of the cell when molecularhalides react with zinc deposited on the anode. Therefore, a separatormust be placed between the two electrodes. However, these membranes havea high internal resistance, and are very expensive. Therefore, a goodion-exchange membrane for a zinc-halogen rechargeable cell has to meetthe following criteria: (1) chemical and electrochemical stability inthe operating environment; (2) mechanical strength and stability undercell operating conditions; (3) electric resistance that can be adjusted,and (4) low production costs.

A variety of membranes have been developed over the years forelectrochemical applications, such as solid polymer electrolytes in fuelcells, electrolyte double-layer capacitors, and rechargeable zinc-halidecells. Sulfonic acids of polydivinylbenzene-styrene based copolymershave been used as electrolytes in fuel cells. Perfluorinated sulfonicacid membranes developed by DuPont and Dow Chemical Company also havebeen used as electrolytes in fuel cells. DuPont's Naflon® membrane isdescribed in U.S. Pat. Nos. 3,282,875 and 4,330,654. Nafion®-typemembranes have high stability and good performance in fuel celloperations. However, they are relatively expensive to produce.

Alternatively, a series of low cost, ion-exchange membranes for PEM fuelcells have been investigated. U.S. Pat. No. 5,422,411 describestrifluorostyrene copolymers that have shown promising performance dataas membranes in PEM fuel cells. Sulfonated poly(aryl ether ketones)developed by Hoechst AG are described in European Patent No. 574,891,A2. These polymers can be crosslinked by primary and secondary amines.When used as membranes and tested in PEM fuel cells, these polymersexhibited only modest cell performance. A series of low cost, sulfonatedpolyaromatic based systems, such as those described in U.S. Pat. Nos.3,528,858 and 3,226,361, also have been investigated as membranematerials for PEM fuel cells. These materials suffer from poor chemicalresistance and mechanical properties that limit their use in PEM fuelcell applications.

Polymer blending is a simple, more feasible technology than methods thatcompound different polymer segments via copolymerization or theformation of inter-penetrating materials. Homogeneous polymer blendsconsist of two polymers that are miscible at the molecular level andcombine the properties of the components to yield a distinct newmaterial. However, very rarely does the blending of polymers result in ahomogenous polymer blend because in general, polymers do not mixhomogeneously, even when they are prepared using the same solvent.

In most cases, Gibbs' free energy of mixing [ΔG=ΔH-TΔS] of polymers is apositive value because the entropy of mixing (ΔS) of high molecularmacromolecules approaches zero when the molecular weight of the polymersis greater than 10,000. Unless the enthalpy of mixing (ΔH) is negativeor at least equal to zero, polymers are not miscible and attempts toblend the polymers results in phase separation in the "blend" resultingin poor mechanical strength, i.e., a non-homogenous "blend" that retainsthe distinct phases of the pure polymers and in most cases, poorinteraction between the phases occurs. Thus, the non-homogenous "blend"falls apart or has a much weaker structure than the original polymers.

Miscibility of polymers occurs in their amorphous regions. If onepolymer in a two polymer blend is a semi-crystalline material, thecrystal structure of the polymer retains its purity in the blend.However, its melting point usually decreases when the two polymers inthe blend are miscible. Therefore, if two polymers are miscible, and oneof the polymers is semi-crystalline, a semi-crystalline polymer blend isformed in which the amorphous structure is miscible. The differentamorphous phases of the two polymers do not separate, but thecrystalline component spreads within the amorphous structure and servesas "crosslink" junctures.

The crosslinking term when applied to crystalline junctures does notrefer to chemical crosslinking as in chemical or radiation treatment.Rather in this context, it refers to what occurs because the crystalsare composed of macromolecules that extend into the amorphous structureand, thus interact and blend with the polymer chains of thenon-crystalline polymers. Therefore, the crystalline structure is tiedup to the amorphous structure in polymer blending by polymer moleculesthat partially take part in the building of the crystal and arepartially amorphous. These polymer molecules take part in the amorphousform and interact with other miscible polymers. For example, it isexpected that a polymer blend, semi-crystalline film will exhibit a muchhigher tensile strength than the theoretical arrhythmic weight averageof the pure polymer component. Also, it is expected that misciblepolymers in a blend will display homogeneity with regard to some desiredproperties such as optical clarity, glass transition temperatures andfor membrane purposes, improved mass transport properties.

Considerable research has been done in attempts to prepare blend polymermembranes. However, only a few membrane systems have been discovered. Y.Maeda et al., Polymer, 26, 2055 (1985) report the preparation of blendmembranes of poly(dimethylphenylene oxide)-polystyrene for gaspermeation. They found this system to exhibit permeation rates unlikethe permeation rates of either of the blend's polymer components.

Poly(vinylidene fluoride), PVF₂, is a hydrophobic polymer that is usedas a membrane in microfiltration and ultrafiltration. Bernstein et al.,Macromolecules, 10, 681 (1977) report that a blend of PVF₂ withpoly(vinyl acetate) increases hydrophilicity of such hydrophobicmembrane, which is needed in order to ultrafiltrate aqueous solutions.They found the macromolecules of the two polymers to be miscible at themolecular level. However, very few scientific tools are provided topredict a blend polymer membrane suitable for use in electrochemicalcells.

In our prior U.S. patent application Ser. No. 08/725,747, published asWO 97/24777 on Jul. 10, 1997, the disclosure of which is incorporatedherein by reference, we disclosed and claimed, inter alia, a polymerelectrolyte membrane containing an ion-exchange polymer membrane, theion-exchange membrane comprising a blend of a homogeneously sulfonatedpoly(phenylene oxide) and poly(vinylidene fluoride), wherein thehomogeneously sulfonated poly(phenylene oxide) had a number averagemolecular weight between about 15,000 and about 10,000,000 and an ioncharge density between about 1 and about 3.9 meq/g, wherein thepoly(vinylidene fluoride) has a number average molecular weight betweenabout 10,000 and about 10,000,000, and wherein the weight ratio ofhomogeneously sulfonated poly(phenylene oxide) to poly(vinylidenefluoride) in the blend was between about 1 to 1 and about 20 to 1.

It is, therefore, an objective of the invention to produce a low cost,easy to prepare ion-exchange polymer membrane with favorable chemicaland mechanical properties for PEM fuel cell and other electrochemicalapplications.

Another object of the invention is to provide an improved solid polymerelectrolyte fuel cell having a high current density, e.g., between 1A/cm² and 2 A/cm² at 0.5 V, using a very low loading electrodeequivalent to a platinum loading of between 0.1 and 0.2 mg/cm² on aplatinum/carbon/PTFE electrode at 30 psi reactant gases.

Another object of the invention is to provide an improved electricaldouble-layer capacitor having a specific capacitance, e.g. between 20farad/cm³ and 60 farad/cm³.

Another object of the invention is to provide a rechargeable zinc-halidecell having a high current and voltage efficiency, e.g., between 80% and100% when tested for 800 cycles (3 h/3 h), and a high energy density,e.g., between 70 mWh/cm³ and 100 mWh/cm³.

Another object is to provide novel homogeneous blends of sulfonatedpoly(phenylene oxide) with poly(vinylidene fluoride).

SUMMARY OF THE INVENTION

The objectives and criteria for the solid electrolyte membrane mentionedabove can be achieved by the practice of this invention. In one aspect,the invention concerns an improved ion-exchange polymer membrane, theimprovement in which the membrane comprises homogeneously sulfonatedpoly(phenylene oxide) blended with poly(vinylidene fluoride), thehomogeneously sulfonated poly(phenylene oxide) having a number averagemolecular weight between about 15,000 and about 10,000,000 and an ioncharge density between about 1 and about 3.9 meq/g, the poly(vinylidenefluoride) having a number average molecular weight between about 10,000and about 10,000,000, the weight ratio of homogeneously sulfonatedpoly(phenylene oxide) to poly(vinylidene fluoride) in the blend beinggreater than about 1 to 49 and less than about 19 to 31.

In another aspect, this invention concerns a solid polymer electrolytemembrane fuel cell containing an ion-exchange polymer membrane aselectrolyte sandwiched between an electrochemically reactive porousanode and cathode, the improvement in which the ion-exchange polymermembrane comprises a blend of a homogeneously sulfonated poly(phenyleneoxide) and poly(vinylidene fluoride), the homogeneously sulfonatedpoly(phenylene oxide) having a number average molecular weight betweenabout 15,000 and about 10,000,000 and an ion charge density betweenabout 1 and about 3.9 meq/g, the poly(vinylidene fluoride) having anumber average molecular weight between about 10,000 and about10,000,000, the weight ratio of homogeneously sulfonated polyphenyleneoxide) to poly(vinylidene fluoride) in the blend being greater thanabout 1 to 49 and less than about 19 to 31, the fuel cell capable ofhaving a current density between about 1 A/cm² and about 2 A/cm² at 0.5V, while using a minimum catalyst loading equivalent to between 0.1mg/cm² and 0.2 mg/cm² of platinum on a platinum/carbonpolytetrafluoroethylene electrode at 30 psi reactant gases, and a celltemperature between 45° C. and 85° C.

Preferably, in the improved ion-exhange polymer membrane and/or thesolid polymer electrolyte membrane fuel cell, the alkali metal of thehomogeneously sulfonated poly(phenylene oxide) of the polymerelectrolyte membrane is Li⁺, Na⁺ or K⁺ ; the number average molecularweight of the homogeneously sulfonated poly(phenylene oxide) is betweenabout 30,000 and about 10,000,000; the ion exchange capacity of thehomogeneously sulfonated poly(phenylene oxide) polymer is between about2 and about 3.5 meq/g; and/or the weight ratio homogeneously sulfonatedpoly (phenylene oxide) to poly (vinylidiene fluoride) in the blend isgreater than about 1/9 and less than about 7/13.

In another aspect, this invention concerns an improved electrolytedouble-layer capacitor containing an ion-exchange polymer membrane aselectrolyte disposed between and in contact with a pair of polarizableelectrodes, the improvement wherein the ion-exchange polymer membranecomprises a blend of a homogeneously sulfonated poly(phenylene oxide)and poly(vinylidene fluoride), the homogeneously sulfonatedpoly(phenylene oxide) having a number average molecular weight betweenabout 15,000 and about 10,000,000 and an ion charge density betweenabout 1 and about 3.9 meq/g, the poly(vinylidene fluoride) having anumber average molecular weight between about 10,000 and about10,000,000, the weight ratio of homogeneously sulfonated poly(phenyleneoxide) to poly(vinylidene fluoride) in the blend being greater thanabout 1 to 49 and less than about 20 to 1, the capacitor capable ofhaving a dielectric constant between about 10 farad/cm³ and about 40farad/cm³ while at a cell temperature between 45° C. and 85° C.

In another aspect, this invention concerns an improved rechargeablezinc-halide cell containing an ion-exchange polymer membrane as a zinccation ion exchange diaphragm positioned between an anode compartmentand a cathode compartment, adapted to substantially entirely preventpassage of bromide ions from the cathode compartment into the anodecompartment, the diaphragm substantially entirely separating the anodecompartment from the cathode compartment, the improvement in which theion-exchange polymer membrane comprises a blend of a homogeneouslysulfonated poly(phenylene oxide) and poly(vinylidene fluoride), thehomogeneously sulfonated poly(phenylene oxide) having a number averagemolecular weight between about 15,000 and about 10,000,000 and an ioncharge density between about 1 and about 3.9 meq/g, the poly(vinylidenefluoride) having a number average molecular weight between about 10,000and about 10,000,000, the weight ratio of homogeneously sulfonatedpoly(phenylene oxide) to poly(vinylidene fluoride) in the blend beinggreater than about 1 to 49 and less than about 20 to 1, the zinc-halidecell capable of having an energy density between about 50 mWh/cm³ andabout 100 Wh/cm³, and a current and voltage efficiency between about 70%and about 100%, while using a minimum catalyst loading equivalent tobetween 0.1 mg/cm² and 0.2 mg/cm² of platinum on a platinum/carbonpolytetrafluoroethylene electrode at 30 psi reactant gases, and a celltemperature between 45° C. and 85° C.

Preferably, in the improved electrolyte double-layer capacitor and/orthe improved rechargeable zinc-halide cell, the number average moleculeweight of the homogeneously sulfonated poly(phenylene oxide) is betweenabout 30,000 and about 10,000,000; the ion exchange capacity of thehomogeneously sulfonated poly(phenylene oxide) polymer is between about2 and about 3.5 meq/g; and/or the weight ratio of homogeneoulsysulfonated poly (phenylene oxide) to poly(vinylidene fluoride) in theblend is greater than about 1/9 and less than about 6 to 1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are scanning electron microscope (SEM) photographsmagnified 2000× of a cross-section of a blend membrane used in both theelectrolyte double layer capacitor and the rechargeable zinc-halide cellof the present invention and a SEM photograph magnified 1000× of asurface of blend membrane useful in the present invention. The blendmembrane comprises 80 wt % sulfonated PPO and 20 wt % PVF₂ (Mw=350,000).

FIGS. 2A and 2B are scanning electron microscope (SEM) photographsmagnified 200× of a cross-section of a blend membrane according to thepresent invention and a SEM photograph magnified 200× of a surface ofthe blend membrane according to the present invention. The blendmembrane comprises 70 wt % sulfonated PPO and 30 wt % PVF₂ (Mw=350,000).

FIGS. 3A and 3B are scanning electron microscope (SEM) photographsmagnified 3000× of a cross-section of a blend membrane according to thepresent invention and a SEM photograph magnified 1000× of a surface ofthe blend membrane according to the present invention. The blendmembrane comprises 50 wt % sulfonated PPO and 50 wt % PVF₂ (Mw=350,000).

FIG. 4 is a polarized light microscope photograph magnified 500× of asurface of a PVF₂ polymer membrane showing its high crystallinestructure.

FIG. 5 is a polarized light microscope photograph magnified 500× of asurface of a blend membrane according to the present invention. Theblend membrane comprises 60 wt % sulfonated PPO and 40 wt % PVF₂.

FIG. 6 is a polarized light microscope photograph magnified 500× of asurface of a blend membrane according to the present invention. Theblend membrane comprises 50 wt % sulfonated PPO and 50 wt % PVF₂(Mw=60,000).

FIG. 7 is a polarized light microscope photograph magnified 500× of asurface of a blend membrane according to the present invention. Theblend membrane comprises 20 wt % sulfonated PPO and 80 wt % PVF₂(Mw=60,000).

DETAILED DESCRIPTION OF THE INVENTION

The sulfonated poly(phenylene oxide) polymer used in this invention hasa chemical structure characterized by the following recurring unit:##STR1## where R₁ and R₂ are each selected from the group ofsubstituents consisting of H, SO₃ H and SO₃ M, wherein n is an integergreater than 40, and M is selected from the group consisting of analkaline metal, an alkaline earth metal, and a transition metal. Whenthe substituents are more frequently SO₃ H and SO₃ M than H, the polymerhas a higher charge density and is more soluble in water. Conversely,when H is more frequently the substituent, the polymer has lower chargedensities.

Suitable alkaline metals include sodium, lithium and potassium; suitablealkaline earth metals include calcium, barium, magnesium and aluminum;and suitable transition metals include chromium and iron. Preferably, R₁and R₂ each include the lithium salt of sulfonic acid and sulfonic acidgroups in the recurring unit. Sulfonated poly(phenylene oxide) polymerssuitable for use in applicants' invention are described in U.S. Pat.Nos. 5,348,569 and 5,364,454, the disclosures of which are hereinincorporated by reference.

It is critical to prepare the sulfonated poly(phenylene oxides) by ahomogeneous process. One homogeneous process involves dissolvingprecursor aromatic polymers in an inert solvent; adding and mixing asulfonation agent at a temperature sufficiently low to avoid anysignificant sulfonation reaction, and raising the temperature of theresulting mixture to cause sulfonation of the precursor aromaticpolymer.

The homogeneously sulfonated poly(phenylene oxide) polymer in salt formis very soluble in common solvents, such as alcohols, ketones, aproticsolvents and mixtures of these solvents with water. The degree ofsulfonation is measured by the ionic charge density, ICD, and expressedin meq/g (which is the milliequivalent of SO₃ -/gram of polymer).

Applicants have discovered that when certain sulfonated poly(phenyleneoxide) polymers as described herein having (1) molecular weights greaterthan about 15,000, preferably greater than about 30,000, more preferablygreater than about 50,000, and less than about 10,000,000, preferablyless than about 1,000,000 and (2) an ion charge density greater thanabout 1.0 meq/g, preferably greater than about 1.7 meq/g, morepreferably greater than about 2.0 meq/g, and less than about 3.9 meq/g,preferably less than about 3.5 meq/g, are blended with certainpoly(vinylidene fluoride)polymers, electrolyte membranes having improvedcharacteristics may be formed. Most preferred is a sulfonatedpoly(phenylene oxide) having an ionic charge density of about 3.0 meq/g.Specifically, the resultant membranes have enhanced selectivity,permeability, and mechanical strength, and are easily crosslinked byγ-ray radiation, ultra violet (UV) radiation and thermal treatment.Moreover, applicants' sulfonated poly(phenylene oxide) polymers can becopolymerized with other polymers.

Applicants also have discovered that a sulfonated poly(polyphenyleneoxide) as defined herein forms a homogeneous blend with poly(vinylidenefluoride) which may advantageously be used as a polymer membraneelectrolyte. Sulfonated PPO is an amorphous cation exchange polymer,while PVF₂ is a hydrophobic thermoplastic polymer. PVF₂ is asemi-crystalline polymer that displays up to 50% semi-crystallinity. Itis soluble in solvents like dimethylsulfoxide (DMSO), N-methylpyrrolidone (NMP), dimethylacetamide, and dimethylformamide (DMF). Itscrystal melting point is between 155° C. and 163° C. and its glasstransition temperature is between -30° C. and -10° C. Therefore, PVF₂ isa non-crosslinked, rubbery polymer where the hard crystalline domainsserve as crosslinking junctures in the blend.

Performance of polymer electrolyte membranes can be measured in terms ofconductivity, current density and/or durability over time. In our priorU.S. patent application Ser. No. 08/725,747, referenced hereinabove, wedisclosed that the performance of polymer electrolyte membranescontaining an ion-exchange membrane in which the ion-exchange polymermembrane comprised a blend of a homogeneously sulfonated PPO and PVF₂,decreased to unacceptable levels as the weight % of PVF₂ in the blendincreased from 50% to 60%. Conductivity, current density and durabilityof the polymer electrolyte membrane decreased to unacceptable levels asthe weight ratio of homogeneously sulfonated PPO to PVF₂ in the blendfell below about 1 to 1. Applicants now have unexpected discovered thatwhen the amount of PVF₂ in the blend exceeds about 62% by weight,performance of the polymer electrolyte membrane containing theion-exchange polymer membrane increases to acceptable levels.

Applicants have found that blends of PVF₂ with sulfonated PPO polymer,as defined herein, exhibit distinct melting points of the crystallinedomain of PVF₂ between 155° C. and 158° C., and have a much highermodulus and tensile strength than sulfonated poly(phenylene oxide) has.Moreover, applicants have discovered that some compositions of suchblends have unexpectedly higher ionic conductivity than sulfonated PPOhas. This is surprising since one would have expected that blendingpoly(vinylidene fluoride) with sulfonated PPO would decrease ionicconductivity. Higher ionic conductivity using a blend membrane accordingto applicants' invention means the fuel cells have better performancethan a fuel cell using a sulfonated PPO membrane.

Applicants also have found that sulfonated PPO and PVF₂ blend membranesconduct ions under an electrical driving force, when comprised of morethan 62% by weight of PVF₂. This is suprising because blend membranescomprised of more than 50% by weight of PVF₂ would not be expected toconduct transport ions or water. PVF₂ is a highly crystallinehydrophobic polymer (FIG. 4) and crystals are impermeable to gases.Thus, one would have expected blend membranes comprised of more than 50%by weight of PVF₂ to be impermeable, even to gases, because theirstructure would be dominated by impermeable crystals of the PVF₂ phaseand interfaces of the crystals.

At weight ratios between about 1 to 1 and about 20 to 1 of sulfonatedPPO to PVF₂, as seen in FIGS. 5 and 6, PVF₂ loses its crystallinity, andthe blend becomes hydrophillic. In contrast, when the weight ratio ofsulfonated PPO to PVF₂ is less than about 19 to 31 and greater thanabout 1 to 49, the blend is high crystalline, as seen in FIG. 7, and theresulting blend membrane is hydrophobic. Applicants have found thatthese crystalline blend membranes provide good barriers to many aqueoussolutes, because of their low affinity to water and suprisingly conductions under an electrical driving force. The crystalline phase in theseblends has a continuous subphase that dominates in the membranestructure, as seen in FIG. 6.

Applicants further have found that there is reduced swelling in blendmembranes according to applicants' invention. It is believed that thePVF₂ in the blend reduces the absorption of water in the blend membranewhen submerged in water. The percent water absorbed (W) is less thanabout 25%.

Applicants' blends are made by dissolving sulfonated poly(phenyleneoxide) in a solution with solvent, dissolving poly(vinylidene fluoride)in a solution with solvent and mixing the solutions together. The blendmembrane then is obtained by casting this mixed solution onto a cleanglass surface with a Doctor knife and drying the solution for a timeperiod sufficient to evaporate essentially all of the solvent, leaving adry, translucent white film having a thickness greater than about 10micrometers (μm), preferably greater than about 40 μm, less than about200 μm and preferably less than about 150 μm.

For applications such as ion-exchange polymer membranes and solidelectrolyte polymer electrolyte membrane fuel cells, the weight ratio ofsulfonated poly(phenylene oxide) to the PVF₂ in the blend according tothis invention is greater than about 1 to 49, preferably greater thanabout 1 to 20, and more preferably greater than about 1 to 9. Also, theweight ratio of sulfonated poly(phenylene oxide) to the PVF₂ in theblend is less than about 19 to 31, preferably less than about 7/13, morepreferably less than about 1 to 3, and even more preferably less thanabout 1 to 4. The sulfonated PPO preferably comprises from 38% to about2% by weight of the blend and PVF₂ comprises from about 98% to 62% ofthe blend. Most preferred are blends where the sulfonated PPO comprisesbetween about 15 wt % and about 20 wt % of the blend, the balance beingthe PVF₂. If desired, the mechanical strength of blend membranes of thisinvention can be further increased by γ-ray radiation, UV radiationand/or by thermal treatment.

For applications such as electrolyte double-layer capacitors andrechargeable zinc-halide cells, the weight ratio of sulfonatedpoly(phenylene oxide) to the PVF₂ in the blend according to thisinvention is greater than about 1 to 49, preferably greater than about 1to 20, and more preferably greater than about 1 to 9. Also, the weightratio of sulfonated poly(phenylene oxide) to the PVF₂ in the blend isless than about 20 to 1, preferably less than about 9 to 1, and evenmore preferably less than about 6 to 1. The sulfonated PPO preferablycomprises from about 2% to about 95% by weight of the blend and PVF₂comprises from about 98% to about 5% of the blend. Most preferred areblends where the sulfonated PPO comprises between 15 wt % and about 85wt % of the blend, the balance being the PVF₂. If desired, themechanical strength of blend membranes of this invention can be furtherincreased by γ-ray radiation, UV radiation and/or by thermal treatment.

Unlike conventional membranes, like Nafion®, which must be pressed ontoelectrodes at elevated temperatures of 120° C. to 150° C. and pressuresof 200 psi to 1,000 psi, the blend membranes of this invention bondeasily to electrodes at room temperature without requiring theapplication of pressure. A good interface is formed between electrodesand the sulfonated PPO membrane or blend membranes of this invention inthe membrane electrode assembly. Electrodes are treated in the usual waywith a solution of proton exchange polymer that can be selected fromNafion® solution dissolved in alcohol and the sulfonated PPO, or othersoluble high charge density cation exchange polymers. The performance ofthe PEM fuel cells made by this invention were compared with theperformance of perfluorinated Nafion® membranes in the same experimentalset up and conditions and were found to be equal or better. The higherdensities obtained with applicants' fuel cell were measured under oneset of conditions. Of course, it is obvious to a skilled person thateven higher current densities can be obtained with applicants' fuel cellunder different conditions. For example, higher current densities can beobtained by using thinner membranes, higher reactant gas pressuresand/or higher temperatures of operation.

The homogeneous blend membrane of sulfonated poly(phenylene oxide) withthermoplastic polymer PVF₂ of the present invention has the followingadvantages: (1) A large series of membranes with different weight ratiosof sulfonated polyphenylene oxide) to PVF₂ can easily be produced; (2)Different copolymers of PVF₂ can be introduced into blends withsulfonated poly (phenylene oxide); (3) Some sulfonated PPO/PVF₂ blendmembranes have higher conductivity than pure sulfonated poly(phenyleneoxide) membranes; (4) Blend membranes have higher flexibility andmechanical strength than pure sulfonated PPO membranes; (5) Blendmembranes have lower swelling ratios in water than pure sulfonated PPO,(6) Blend Membranes have enhanced ion conductivity because of membranehydrophilicity, (7) Blend Membranes have decreased electric resistance,because of ion immobilization (due to sulfonated PPO) on the membranematrix (PVF₂), and (8) Blend Membranes have a resistance that can beadjusted easily by changing the ratio of the two blend components andmembrane thickness. Therefore, these membranes can be used as: (1)polymer electrolyte membranes for hydrogen/oxygen electrochemical fuelcells; (2) electrode separators for secondary batteries; (3)ion-exchange membranes for electrodialysis, in which membranes areemployed to separate components of an ionic solution under the drivingforce of an electrical current; (4) membranes for gas separation andpervaporation due to the enhanced selectivity and permeability ofhomogeneously sulfonated poly(phenylene oxide); (5) electrode separatorsfor capacitors; and (6) ion-exchange membranes for Zinc-halide cells.

The following examples illustrate applicant's invention, but should notbe construed as limiting the invention:

EXAMPLE 1

A light yellow sulfonated PPO polymer in the Li⁺ form (M_(w) =50,000)was immersed in 1N HCl solution for several hours at room temperature.This step exchanges Li⁺ with H⁺ in the SO₃₋ group. The polymer was thenwashed carefully in D.I. water to rinse the excess acid. The wetsulfonated PPO in the H⁺ form was put in a vacuum oven for 24 h at 40°C. The sulfonated PPO was then dissolved in dimethylformamide (DMF) toform a 20 wt % solution. A 20 wt % solution of poly(vinylidenefluoride), PVF₂ (M_(w) =60,000), in DMF was prepared separately. Then,2.55 g of sulfonated PPO-DMF solution and 0.45 g of PVF₂₋ DMF solutionwere blended by mixing the two solutions at room temperature for 1 h.This blend has an 85:15 weight ratio of sulfonated-PPO:PVF₂. This blendsolution was poured onto a clean glass plate surface and cast by aDoctor knife. This was then placed in a chamber under dry air flow for48 h to evaporate most of the DMF. The final membrane was a dry,translucent, white with a 50 μm thickness. The ICD of this membrane was2.9 meq/g. This membrane is suitable for use in electrolyte double-layercapacitors, and rechargeable zinc-halide cells.

EXAMPLE 2

20 wt % sulfonated-PPO Li⁺ form polymer (M_(w) =50,000) was dissolved inDMF and 20 wt % of PVF₂ (M_(w) =60,000) was dissolved in DMF,separately. Then, the two solutions were mixed in a weight ratio ofsulfonated PPO to PVF₂ of 80:20. This blended solution was stirred atroom temperature for 1 h. The blend solution was poured onto a cleansurface glass plate, and cast by a Doctor knife. The cast solution thenwas placed in a chamber under dry air for 48 h. After the membrane haddried, it then was placed into 0.5 N HCl solution for exchange of Li⁺ tobe converted to proton form. The cast membrane was 55 μm thick and inthe wet state, has a conductivity of 0.22 S/cm at 45° C. This membraneis suitable for use in electrolyte double-layer capacitors, andrechargeable zinc-halide cells.

EXAMPLE 3

25 wt % of sulfonated PPO Li⁺ form polymer (M_(w) =50,000) was dissolvedin isopropanol. No second polymer was added. The sulfonated PPO membranewas cast by a Doctor knife on a clean surface glass plate. The membranewas dried in dry air atmosphere for 48 h and then put in an oven at 70°C. for 24 h. The sulfonated membrane was transparent with a lightyellow-brown color. The thickness of the membrane was 120 μm. Themembrane then was placed in 0.1 N HCl solution for 1 h. The ICD of thismembrane was measured as 3.0 meq/g. The swelling ratio of this membranein water was 25% at 30° C. and 31% at 80° C. Conductivity of themembrane at 45° C. was 0.016 S/cm.

EXAMPLE 4

The same procedure was employed as in Example 3, except that themembrane was crosslinked by γ-ray radiation.

EXAMPLE 5

The same procedure was employed as in Example 3, except that the solventused was DMF.

EXAMPLE 6

The same procedure was employed as in Example 3, except that themembrane was subjected to crosslinking by heat treatment at 80° C. for 5minutes.

EXAMPLE 7

The same procedure was employed as in Example 3, except that themembrane was exposed to UV radiation for 30 minutes.

EXAMPLE 8

The same procedure was employed as in Example 1, except that the weightratio of sulfonated PPO to PVF₂ was 75:25. This membrane is suitable foruse in electrolyte double-layer capacitors, and rechargeable zinc-halidecells.

EXAMPLE 9

The same procedures were used as in Example 1, except that the weightratio of sulfonated PPO to PVF₂ was 70:30. This membrane is suitable foruse in electrolyte double-layer capacitors, and rechargeable zinc-halidecells.

EXAMPLE 10

The same procedures were used as in Example 1, except that the weightratio of sulfonated PPO to PVF₂ was 65:35. This membrane is suitable foruse in electrolyte double-layer capacitors, and rechargeable zinc-halidecells.

EXAMPLE 11

The same procedures were used as in Example 1, except that the weightratio of sulfonated PPO to PVF₂ was 50:50. This membrane is suitable foruse in electrolyte double-layer capacitors, and rechargeable zinc-halidecells.

EXAMPLE 12

A sulfonated PPO polymer in Li⁺ form (Mw=50,000) was dissolved indimethylformamide (DMF) to form a 20 wt % solution. A 20 wt % solutionof high molecular weight PVF₂ (Mw=350,000), in DMF was preparedseparately. Then 8 g of sulfonated PPO-DMF solution and 2 g of PVF₂ -DMFsolution were blended by mixing the two solutions at room temperaturefor 0.5 h. This blend had an 80:20 weight ratio of sulfonated PPO:highmolecular weight PVF₂. The blend solution was poured onto a cleansurface glass plate, and cast by a Doctor knife. Then the blend membranewas placed in a chamber under dry air for 48 h. The blend membrane wasplaced into a 0.5 N HCl solution for exchange of Li⁺ and conversion toproton form. This membrane is suitable for use in electrolytedouble-layer capacitors, and rechargeable zinc-halide cells.

EXAMPLE 13

The same procedure was used as in Example 12, except the weight ratio ofsulfonated PPO to High molecular weight PVF₂ was 70:30. This membrane issuitable for use in electrolyte double-layer capacitors, andrechargeable zinc-halide cells.

EXAMPLE 14

The same procedure was used as in Example 12, except the weight ratio ofsulfonated PPO to PVF₂ (Mw=350,000) was 50:50. This membrane is suitablefor use in electrolyte double-layer capacitors, and rechargeablezinc-halide cells.

EXAMPLE 15

The same procedure was used as in Example 12, except the weight ratio ofsulfonated PPO to PVF₂ (Mw=350,000) was 40:60. The fuel cell preparedusing this membrane tested for 50 hours at 80° C. without decreasingperformance. The conductivity of this membrane is 0.027S/cm at 80° C.and its water uptake is 2.9 n H₂ O,/charge. The fuel cell has an openvoltage of 0.85V and a current density of 0.4 A/cm² at 80° C. Thismembrane is suitable for use in electrolyte double-layer capacitors, andrechargeable zinc-halide cells.

EXAMPLE 16

Sulfonated-PPO in the Li⁺ form, and with a high charge density (ICD=3.7meq/g), was dissolved in DMF to form a 20 wt % solution. A 20 wt %solution of PVF₂ (M_(w) =60,000) in DMF was prepared separately. The twosolutions then were blended in a weight ratio of sulfonated PPO to PVF₂of 37:63. This blended solution was stirred at room temperature for 1 h.The blend solution was poured onto a clean surface glass plate and castby a Doctor knife. This cast solution then was placed in a chamber underdry air for 48 h to evaporate most of the DMF. After drying, themembrane was placed into 0.5 N HCl solution for exchange of Li⁺ to beconverted to proton form. The cast membrane was 70 μm thick and in thewet state. This polymer electrolyte blend membrane then was placedbetween two electrodes at ambient temperature and pressure. Theresulting PEM cell displayed a current density of 1.0 A/cm² at 0.5V.This membrane is suitable for use in PEM fuel cells, electrolytedouble-layer capacitors, and rechargeable zinc-halide cells.

EXAMPLE 17

A double-layer capacitor was fabricated from activated carbon paperelectrodes and a sulfonated PPO and PVF₂ blend membrane, as theseparator. A 30 wt % solution of H₂ SO₄ was employed as the electrolyte.Solutions of sulfonated PPO and PVF₂ in DMF were prepared separately,blended in a weight ratio of sulfonated PPO to PVF₂ of 10:90, and mixedat room temperature for 1 h. This blend solution then was poured onto aclean glass plate surface and cast by a Doctor knife. This cast solutionwas then submerged in D.I. water to form a porous membrane. Twocarbonized phenolic resin carbon paper electrodes and the porousmembrane were immersed into a 30 wt % H₂ SO₄ solution overnight. Themembrane then was sandwiched between the two carbon paper electrodes, asa separator. The membrane had a thickness of about 70 μm, and theapparent electrode area was 1.0 cm². A capacitance of 1.8 Fcorresponding to 20 farad/cm³ was observed, when discharged with a100-ohm resistor.

EXAMPLE 18

The same procedure was employed as in Example 17, except that a weightratio of sulfonated PPO to PVF₂ of 2:98 was used to make themembrane/separator for the double-layer capacitor. A capacitance of 1.2F corresponding to 14 farad/cm³ was observed.

EXAMPLE 19

A non-flow zinc bromine cell is constructed as follows: A paste preparedfrom 20% polyacrylamide and 80% carbonized phenolic resin carbon isplaced into a cathode compartment. 5 M ZnBr₂ /KCl polyacrylamide hydrogel is placed into an anode compartment. A thin sulfonated PPO/PVF₂blend membrane is sandwiched between the cathode and anode compartments,as the separator.

The blend membrane/separator was prepared using the following process:sulfonated PPO was dissolved in DMF to form a 20 wt % solution. A 20 wt% solution of PVF₂ was prepared separately. These solutions were blendedin a weight ratio of sulfonated PPO to PVF₂ of 40:60, and mixed at roomtemperature for 1 h. This blend solution was poured onto a clean glassplate surface and cast by a Doctor knife. The cast solution then wasplaced in a chamber under dry air flow for 48 h to evaporate most of theDMF.

The cell had a cross-sectional area of 30 cm². The capacity of the cellwas 5.8 A h with an energy density of 83 mWh/cm³. This cell was testedfor 800 cycles (3 h/3 h) with a current and voltage efficiency of about95%.

EXAMPLE 20

To make a membrane/separator for a non-flow zinc bromine cell, the sameprocedure was employed as in Example 3, except that a weight ratio ofsulfonated PPO to PVF₂ was 30:70. The current and voltage efficiency ofthe non-flow zinc bromine cell were about 93%.

What is claimed is:
 1. An ion-exchange polymer membrane comprising a blend of a homogeneously sulfonated poly(phenylene oxide) and poly(vinylidene fluoride), the homogeneously sulfonated poly(phenylene oxide) having a chemical structure characterized by the following recurring unit: ##STR2## wherein R₁ and R₂ are each selected from the group consisting of H, SO₃ H and SO₃ M; and M is a metal selected from the group consisting of an alkali metal, an alkaline earth metal and a transition metal, and n is an integer greater than 40, the homogeneously sulfonated poly(phenylene oxide) having a number average molecular weight between about 15,000 and about 10,000,000 and an ion charge density between about 1 and about 3.9 meq/g, the poly(vinylidene fluoride) having a number average molecular weight between about 10,000 and about 10,000,000, the weight ratio of homogeneously sulfonated poly(phenylene oxide) to poly(vinylidene fluoride) in the blend being between greater than about 1/49 and less than about 19/31.
 2. The polymer electrolyte membrane of claim 1, wherein in said homogeneously sulfonated poly(phenylene oxide), said alkali metal is Li⁺, Na⁺ or K⁺.
 3. The polymer electrolyte membrane of claim 1, wherein said homogeneously sulfonated poly(phenylene oxide) has a number average molecular weight between about 30,000 and about 10,000,000.
 4. The polymer electrolyte membrane of claim 1, wherein said homogeneously sulfonated poly(phenylene oxide) polymer has an ion exchange capacity between about 2 and about 3.5 meq/g.
 5. The polymer electrolyte membrane of claim 1, wherein said weight ratio is greater than about 1/9 and less than about 7/13.
 6. In a solid polymer electrolyte membrane fuel cell containing an ion-exchange polymer membrane as electrolyte sandwiched between an electrochemically reactive porous anode and cathode, the improvement in which the ion-exchange polymer membrane comprises a blend of a homogeneously sulfonated poly(phenylene oxide) and poly(vinylidene fluoride), the homogeneously sulfonated poly(phenylene oxide) having a chemical structure characterized by the following recurring unit: ##STR3## wherein R₁ and R₂ are each selected from the group consisting of H, SO₃ H and SO₃ M; and M is a metal selected from the group consisting of an alkali metal, an alkaline earth metal and a transition metal, and n is an integer greater than 40, the homogeneously sulfonated poly(phenylene oxide) having a number average molecular weight between about 15,000 and about 10,000,000 and an ion charge density between about 1 and about 3.9 meq/g, the poly(vinylidene fluoride) having a number average molecular weight between about 10,000 and about 10,000,000, the weight ratio of homogeneously sulfonated poly(phenylene oxide) to poly(vinylidene fluoride) in the blend being greater than about 1/49 and less than about 19/31, the fuel cell capable of having a current density between about 1 A/cm² and about 2 A/cm² at 0.5 V, while using a minimum catalyst loading equivalent to between 0.1 mg/cm² and 0.2 mg/cm² of platinum on a platinum/carbon polytetrafluoroethylene electrode at 30 psi reactant gases, and a cell a temperature between 45° C. and 85° C.
 7. The polymer electrolyte fuel cell of claim 6, wherein said homogeneously sulfonated poly(phenylene oxide) has a number average molecular weight between about 30,000 and about 10,000,000.
 8. The polymer electrolyte fuel cell of claim 6, wherein said homogeneously sulfonated poly(phenylene oxide) polymer has an ion exchange capacity between about 2 and about 3.5 meq/g.
 9. The polymer electrolyte fuel cell of claim 6, wherein said weight ratio is greater than about 1/9 and less than about 7/13. 